Use "one" when you need a neutral pronoun (neither male nor female). The use of "one" is found in formal English, but in normal, everyday English, it's not popular. Many Americans choose to use the pronoun "you" instead of "one" The pronoun "one" refers to "a person."

subject = one

One needs to work at staying health.

A person needs to work at staying healthy.

You need to work at staying healthy.

one = a person

you = a person

object = one

The view of the Grand Canyon leaves one with a lasting impression.

possessive adjective = one's

It's one's responsibility as a citizen to vote when there are elections.

possessive pronoun = 0

X

reflexive pronoun = oneself

One needs to remind oneself of what is really important in life, such as one's children.

(This sounds a little too formal, but you might hear "oneself" used in this way.)

Remember:

1. The use of "one" is very formal. It's rarely used in spoken English, but it's useful in written English. In any case, most people choose not to use it.

2. The word "one" is also a noun. It can be used for a person or a thing. The plural form of "one" is "ones." This is not the same as the pronoun "one."

3. It's possible to use "one" as an adjective. In the expression, "They're one and the same," the speaker wants to indicate that two people or two groups are very similar. This word also goes before words like "people," "nation," "purpose," "world," etc., to emphasize unity or wholeness. The people of Iraq no longer think of themselves as one people, but rather as separate groups divided along religious and ethnic lines.